1. Field of the Invention
The present invention relates to a beam splitter for a camera and a light measuring device for a camera in which the beam splitter is used.
2. Description of the Prior Art
Until now many systems have been known for the light measuring device for single lens reflex cameras, and a light measuring device of the type shown in FIG. 1 is disclosed in U.S. patent application Ser. No. 371,017 filed Apr. 22, 1982, now U.S. Pat. No. 4,443,081.
In the case of the above proposed device, a quick return mirror 3 which is brought out of the optical path when taking a picture is arranged in front of the film plane 2 on the optical axis O of the photographic lens 1, and the quick return mirror 3 has a semi-transmitting portion 4 for passing light to the light measuring optical system and behind the mirror a submirror 5 for reflecting the light downwards is arranged, formed as one unit with the quick return mirror 3. Further above the quick return mirror 3, the focus plate 6, the condenser lens 7 and the pentagonal prism 8 are provided in sequence along the path of the light, while on the surface of the pentagonal prism at which the light is emitted an eye piece lens 9 is provided so as to enable viewing of an object.
The light beam L1 for light measurement passing through the semi-transmitting portion 4 of the quick return mirror 3 and incident upon the submirror 5 is reflected downwards by the submirror 5 and led to the light measuring optical system 10. The light measuring optical system 10 consists of a beam splitter 11 having a fine diffraction grating on the surface and a photoelectric transducing element 12 arranged at the position on the end surface at which the light is condensed. The light beam L1 is effectively led inside by the fine diffraction grating on the surface of the beam splitter 11, repeatedly and totally reflected in the inside, condensed at the end surface and incident upon the photoelectric transducing element 12. Further, the quick return mirror 3 is raised at the time of flash light photographing so that the photographic light beam L from the photographic lens 1 is led to the beam splitter 11 as the light beam L1' diffused on the film surface 2 as is shown in a dotted line.
Although in the case of the light measuring optical system as mentioned above, a compact light measuring optical system can be realized with the thin plate-like beam splitter 11 by effectively using a fine diffraction grating, the system has a defect in that change-over of the distribution of the light sensitivity to a degree required with current technology cannot be achieved.
Further, quite recently a camera, particularly a single lens reflex cameras having automatic focus adjusting means have been built and developed. In the case of most automatic focus adjusting device for single lens reflex cameras, a focus detecting element such as CCD (change coupled device) for sensing the object light via the photographic lens is arranged at the position optically equivalent to the film plane in such a manner that the in-focus signal is produced according to the output signal from the focus detecting element so as to move the photographic lens in order to obtain correct focusing. Taking the arrangement of other members and other units of the single lens reflex camera into consideration, it is convenient to arrange this focus detecting element at the bottom of the mirror box which bears the inclined rotatable mirror.
In the case of a light measuring device in which the beam splitter 11 of the light measuring device shown in FIG. 1 is provided, the focus detecting element 14 has to be arranged beneath the beam splitter 11.
However, the above-mentioned beam splitter 11 is constructed so as to direct the object light reflected by the submirror to the light measuring element 12 so that there is caused a problem in that the focus detecting element 14 cannot receive a sufficient amount of light and that an accurate focus signal cannot be obtained from the focus detecting element 14. Further, it is necessary that the object light with less noise should be directed to the focus detecting element, but there is a fear in the conventional art that a part of the light reflected from the diffraction structure would be incident on the focus detecting element as noise light.
The beam splitter of the present invention distributes the light to the two control systems of the light measuring device and the focus detecting device of the camera without causing noise light, while the conventional beam splitter does not have a distribution function of the object light such as mentioned above.
For example, in U.S. Pat. No. 4,172,646 a diffraction grating is used in the light measuring device for a camera, wherein the object light is directed along the direction of the light measuring element by making use of the total reflection, so that the structure is not sufficient for leading the object light to the focus detecting element.
Further, in the case of the device for detecting light and/or focusing by making use of a diffraction grating as mentioned above, it is necessary to control the amount of the object light incident on the light measuring device by changing over the distribution of the light measuring sensitivity according to the brightness distribution of the object light in the picture frame, whereby in the case of the conventional light measuring system, particularly using the diffraction grating the development of techiques for changing over the sensitivity distribution is delayed.